The CO2 in the atmosphere is constantly captured and converted through photosynthesis by various types of plants to produce human or animals' foods, such as sugar, starch, rice, wheat, potato, yam, grass, algae, vegetable, tree leaves, fruits et. al. After these foods are digested by human or animals, their excreta or the excess kitchen and agriculture's wastes can be used to produce biogas by an anaerobic digester, and the subsequent use of this biogas as a fuel for a combined heat and power (CHP) generator is realistically just returning the captured CO2 back into the atmosphere. In other words, the biogas is actually a short term recycled product of converting CO2 from one type of energy material into various other types of energy material and/or biomass, and the net carbon emission from a biogas CHP generator is zero. Therefore, using biogas as a fuel for a heat and power generator can reduce worldwide net carbon emission.
Anaerobic Digester:
By using the fermentation process in the absence of oxygen at about 38° C. (i.e. about 100° F.), an anaerobic digester can be used to produce biogas, odorless organic water, and odorless solid fertilizer from the waste materials. The biogas produced from the digester consists of 55-60% methane, 40-45% CO2, 0.4-1.2% N2, 0.0-0.4% O2 and 0.02-0.4% H2S (americanbiogascouncil.org).
Combustible Reformed Gas and IC Engine (i.e. Reformate Engine):
A lot of Internet videos and documents have showed that a reformate gas containing hydrogen and CO can successfully be used as the sole fuel for an internal combustion engine. For example, the reformate gas produced from the partial combustion of woods had been used for several years in Germany during World War II (https://www.youtube.com/watch?v=bIWVB-JpdEk). Also, a small California company, All Power Laboratory, has designed a small gasifier to generate combustible H2 and CO gas from waste woods and husk, and has used this gas as the only fuel for a commercial IC engine. It has been reported that the combination of their gasifier, an IC engine and an electrical generator can generated enough electricity for their own facility from the waste biomass.
U.S. Pat. Nos. 7,028,644 and 7,225,787, herein incorporated by references, had used an on-board Plasmatron converter to produce the H2 and CO reformate from hydrocarbons (HC) and bio-fuels. This reformate gas could then be mixed with extra air and fuel to become a lean burn fuel mixture as the engine's sole fuel, and the engine can then be operated under higher compression ratio. As the results, this on-board Plasmatron reformer could increase gasoline engine's thermal efficiency by 20-25%, could reduce gasoline engine's NOx emission up to 90% and could reduce diesel engine's emission by 90% (Diamond and Cohn, Office of Transportation Technologies, press release 12/2001).
Flexible-Fuel Hydrogen Generator for IC Engines and Gas Turbines:
U.S. Pat. No. 9,440,851, herein incorporated by reference, describes the use of an on-board catalytic reformer to produce H2 and CO reformate gas from various vaporized hydrocarbons and/or bio-fuels and, thus, it can be used to replace the Plasmatron reformer to generate H2 for the IC engine. Since this reformer uses the Pt group metal catalysts to accelerate the reactions, and it also includes the necessary devices and the control software for automatic operation of the generator, this catalytic reformer is fundamentally very different in equipment design and the operating procedures from those of the homogeneous (i.e. non catalytic) partial oxidation gasifier or Plasmatron.
The reformed gas so produced can be cooled, dried and compressed to 1-100 atmospheres, and can be stored in one or several high pressure vessels. This stored combustible reformate can then be used as a sole fuel for a reformate IC engine/gas turbine, which will drive a generator to produce electricity, or it can be combined with extra air and extra fuel to become a lean burn fuel mixture for the IC engine/gas turbine. By utilizing this lean combustion process at a higher compression ratio, complete combustion reactions inside the engine cylinder can successfully increase the engine/gas turbine's thermal efficiency and can also reduce its pollution emission.
Removing Hydrogen Sulfide from Biogas and the Common Industrial Desulfurization Process:
The biogas from the anaerobic digester contains 0.02-0.4% hydrogen sulfide (H2S). After complete combustion inside an engine cylinder, the H2S will be converted into SOx, which will combine with moisture/steam and produce acid in the exhaust gas. Therefore, to avoid corrosion to the downstream IC engine and other equipment, this H2S biogas must be removed before using it as the direct IC engine/gas turbine's fuel.
Currently, the common low temperature sulfur removal processes for biogas are water wash, caustic soda solution wash (solutions such as sodium hydroxide, potassium hydroxide, calcium hydroxide, mono ethanolamine etc.), iron oxides, iron sponge, activated carbon, molecular sieve, silica gel, activated alumina, bio-bacteria sulfur removal technology, pressure swing adsorption process etc. However, if the above adsorbents are used, they either are non-regenerable or cannot be regenerated repeatedly for long term application. Therefore, they have waste disposal problem once they reach their saturation capacity (Allegue and Hinge, Danish Technological Institute, December 2014).
One common commercial industrial sulfur removal technology is to use the catalytic hydro-desulfurization process, which can convert any organic sulfur compounds catalytically with the recycled hydrogen to produce H2S over a Co—Mo/Al2O3 catalyst at 500° C. and 100 atmospheres and, then, the process will use ZnO adsorbent to remove the H2S from the gas stream. In 1985, U.S. Pat. No. 4,522,894, herein incorporated by reference, had described a new catalytic autothermal reforming (ATR) process to produce hydrogen and CO over a monolithic Pt/Pd/Al2O3 catalyst from a diesel oil, air and steam fuel mixture at O2/C ratio=0.378, H2O/C ratio=2.57, 1 atmosphere and a temperature <1000° C. Under this operating condition, all organic sulfur compounds (about 0.12% sulfur) in the commercial diesel oil were converted into hydrogen sulfide at about one atmosphere, and the produced H2S in the reformate was then removed by a downstream high temperature ZnO adsorbent bed. In addition, U.S. Pat. No. 7,128,768, herein incorporated by reference, has described the use of a mini catalytic reformer or other H2 generator to produce dry hydrogen to convert all sulfur compounds in the hydrocarbons into H2S, and then removed the H2S by a downstream ZnO bed. However, the ZnO adsorbent needs to be replaced once it reaches its saturation capacity as shown in US 2004/0178124 A1, herein incorporated as reference.
U.S. Pat. No. 7,074,375, herein incorporated by reference, describes a catalytic oxidation process of converting all sulfur compounds into SOx (i.e. SO2 and SO3) with an oxidation catalyst at a O2/C ratio between 0.01 to 0.04, and at a preferred temperature between 250 to 400° C. The SOx produced can be removed by the downstream adsorbents such as alkali metal oxides, alkali earth metal oxides and/or base metal oxides (i.e. oxides of Ni, Fe, Cu, Zn supported on porous SiO2 or Al2O3). Subsequent U.S. Pat. Nos. 9,034,527, 9,640,822 and 9,515,338, herein incorporated by references, have advanced this catalytic oxidation process to remove the organic sulfur compounds from various hydrocarbons. Briefly, this patented sulfur removing process is to control the O2/C ratio below 0.04 over an oxidation catalyst to produce selectively SOx, but not to hydrogenate the sulfur compounds into H2S over a Pt group metal hydrogenation catalyst. As shown in the patent, H2S will be produced if O2/C ratio of the feed gas is greater than 0.04, especially above 0.08.
U.S. Pat. Nos. 7,820,037, 8,236,262, 8,308,848 and 9,421,516, herein incorporated as references, describe the preparation methods for Cu—Zn—Al—Ni/Fe, Ni—Cu—Al—Na—Zn, Ni/Al and Al/Ni/Zn oxide catalysts, which are used as adsorbents for removing sulfur compounds from various hydrocarbon fuels.
U.S. Pat. Nos. 4,552,733, 4,780,447, 4,939,113, 5,024,985, 5,196,390 and 7,871,459, herein incorporated as references, have shown that all sulfur compounds in the gasoline are converted into SO2 after the engine combustion, and the SO2 is stored by the downstream catalytic converter's oxygen storage component, which is mainly cerium oxide, as sulfate, sulfide and/or sulfur radicals. The stored sulfur is released as H2S later by the next engine's rich operating mode. Therefore, it has been demonstrated that the Pt group catalyst, which also contains one or more oxides of Ni, Fe, Cu, Mn, Ce, Ce/Zr and mixture thereof, is able to remove H2S from the exhaust gas by adsorption/reaction during the engine's rich operating mode and, then, be regenerated by converting the stored sulfur into SO2 with the exhaust gas during the next engine's lean operation mode, which contains a small % of excess O2.
U.S. Pat. No. 7,901,566, herein incorporated as reference, describes the use of a catalyst in a pre-reformer to remove sulfur compounds from the hydrocarbon (HC) fuels with steam. The catalyst is a Pt/Rh supported on a non-sulfating SiO2—ZrO2 oxide, which also consists at least one member of Ni, NiO, Mn, MnO, Fe, FeO and Fe2O3. The deactivated catalyst can then be regenerated either with O2 or with a gas mixture consisting of O2, steam and HC to remove the adsorbed sulfur on the catalyst surface as SO2.